We have discovered a better way of coupling a coiled tension spring to the components of a block and tackle window balance system. In this environment, spring couplers often fail under the large stresses they must withstand. The block and tackle spreads the spring force over a much longer distance than the spring is stretched, and the spring provides a much larger spring force that is reduced by the block and tackle as it is spread over the distance traveled by a window sash. The spring force exceeds the balance force applied to the sash by a factor of the number of working cord reaches in the block and tackle, and the couplers that connect the ends of the spring to the components of the block and tackle must withstand this large spring force.
Dinsmore U.S. Pat. No. 3,358,404 provides a good example of a block and tackle window balance system using a coiled tension spring. The block and tackle, with its pulleys and cord reaches, is confined in a channel along with the spring; and one of these assemblies is arranged at each side of each sash. The spring is tensioned between an anchorage fixed in the channel and a block and tackle that is movable within the channel. The spring stretches and contracts over a short distance under a large force that the block and tackle reduces and spreads over a longer working distance that the sash travels.
A popular way of coupling a coiled tension spring to components of a block and tackle window balance system has been to bend hooks in the terminal coils at each end of the spring. These bends weaken the terminal coils so that the spring is likely to fail by breaking off one of its end hooks. Alternative spring couplings have included coiling a spring tightly around a cylinder, threading a connector into the end coils of a spring so that the pitch of the thread matches the pitch of the end coils, and bending spring ends to form pins inserted into retainer holes. Any sharp bend in a spring end weakens the spring at that point and is unsatisfactory, and spring grips on cylinders or threaded connectors have required a different coupler for each size of spring and have been too weak to withstand the spring force of a block and tackle balance system.
Decker U.S. Pat. No. 2,851,721 on a window balance system suggested that the sheath surrounding a spring operating in both torsion and tension be dented inward to force coils of the spring into a window in a hollow holder tube. Then the discontinuities in the sheath, the dented spring coils, and the holder tube would help retain the spring against axial movement. Our spring coupler also indents spring coils into an opening in a hollow tube; but it does not involve a surrounding sheath, does not apply to a spring working partially in torsion, and does not lack the strength to resist the high tension forces exerted by block and tackle window balance springs.